Rapid investment casting or molding method

ABSTRACT

An investment casting or molding method comprises encasing a consumable pattern in a no-bake material that hardens without the application of heat. The encased pattern is then transformed to a fluid state and evacuated, thereby forming a mold cavity. Preferably, the pattern is coated with a refractory material prior to encasement, so that the resulting mold cavity is lined with the refractory material. Coating the pattern with one and only one layer of refractory material is preferred, resulting in a refractory coating that is not self-supporting. In another embodiment, a refractory-coated consumable pattern is located in a container, and a granular backing material is packed in the container around the coated pattern. The pattern is then transformed to a fluid state and evacuated to form a refractory-lined mold cavity in the container. The refractory coating again preferably comprises one and only one layer of refractory material, thereby resulting in a coating that is not self-supporting.

TECHNICAL FIELD

[0001] This invention relates to a method for producing cast or molded parts by investment casting and, more particularly, to a method for rapidly making a casting mold and performing such casting.

BACKGROUND

[0002] Typical investment casting involves the use of a consumable three-dimensional pattern to create a mold cavity. For example, in a lost wax process, a wax pattern is first immersed in a slurry of aqueous ceramic or other refractory material, and the wetted surface of the pattern is then covered with a stucco coat of sand, powdered ceramic material, or the like. The stucco-coated surface is then permitted to air dry. This immersion, stucco coating, and drying process is repeated several times to form a self-supporting, multi-layer ceramic shell of substantial thickness encasing the pattern. After the wax pattern is encased in the ceramic shell, heat is applied to the ceramic-encased pattern to melt the wax, which is evacuated from the ceramic shell through an opening in the shell. As a result, a mold cavity is formed having a smooth ceramic surface. Molten metal or other casting or molding material can then be poured or otherwise introduced into the cavity to produced a cast or molded part, which is generically termed a casting for purposes of this invention. Finally, the ceramic shell is broken away from the casting, thereby destroying the mold.

[0003] Other consumable materials other than wax are also known for use in forming patterns for investment casting. For example, lost foam processes are also known in which the pattern encased in ceramic material is formed from a consumable foam plastic, such as polystyrene or the like. In addition, U.S. Pat. No. 4,844,144 describes a casting process that utilizes an expendable stereolithographic pattern. However, apart from the nature of the pattern material, conventional investment casting processes generally rely on the application of multiple layers of stuccoed ceramic material. As a result, creating investment molds using known investment processes can be very time-consuming and costly.

[0004] Other methods for creating casting molds are also known in the art, such as the use of green sand to produce two mold parts or flasks, which when abutted against one another, produce a complete mold cavity. Green sand is well known in the art and typically consists of a silica or other sand and a natural binder, such as clay. While fast to create, green sand molds result in casting with parting lines along the interface between the two mold parts. In addition, green sand molds may not be suitable for casting intricate part requiring a high surface finish or intricate surface detail. Air set sand may also be used, which provides a better surface finish and better dimensional control than a green sand mold but is time consuming to create. A chemically-bonded, self-setting sand mixture, known as “no-bake” sand or a “no-bake” binder, is also known in the art for making molds in a manner similar to the green sand process, whereby two mold parts or flasks are formed from no-bake sand and positioned together to form the complete mold. While such no-bake sand process provides certain advantages over green sand, the no-bake sand has a limited bench life and the process requires that the reusable patterns be kept in good condition.

[0005] This invention is directed to overcoming one or more of the problems described above.

SUMMARY OF THE INVENTION

[0006] In one aspect of this invention, a method for producing a casting is disclosed. The method comprises providing a pattern that is formed from a medium that is transformable to a fluid state by application of heat and applying a coating of refractory material to the pattern. The refractory-coated pattern is encased in a material that hardens without application of heat, and after the encasing material is hardened, the pattern is transformed to a fluid state by applying heat thereto. The fluid pattern medium is evacuated from the encasing material, thereby producing a refractory-lined mold cavity in the encasing material. A fluid casting medium is supplied into the mold cavity and permitted to achieve a solid state.

[0007] In another aspect of this invention, a method for producing a casting comprises providing a pattern formed from a medium that is transformable to a fluid state by application of heat applying a coating of refractory material to the pattern. The refractory-coated pattern is located in a container, and a free granular backing material is packed in the container around the refractory-coated pattern. The pattern is transformed to a fluid state by applying heat thereto and the fluid pattern medium is evacuated from the container, thereby producing a refractory-lined mold cavity in the container. A fluid casting medium is supplied into the mold cavity and permitted to achieve a solid state.

[0008] In yet another aspect of this invention, a method for producing a casting comprises providing a pattern formed from a medium that is transformable to a fluid state by application of heat. The pattern is encased in a single layer of a material that hardens without application of heat to thereby form a one-piece, self-supporting shell around the pattern, and after the encasing material is hardened, the pattern is transformed to a fluid state by applying heat thereto. The fluid pattern medium is evacuated from the encasing material, thereby producing a mold cavity in the encasing material. A fluid casting medium is supplied into the mold cavity and permitted to achieve a solid state.

[0009] Other features and aspects of this invention will become apparent from the following description.

DETAILED DESCRIPTION

[0010] In accordance with this invention, a casting is formed by first providing a consumable pattern of the object to be cast. The consumable pattern may be formed from wax, stereolithographic material, low melting point tin-bismuth, frozen liquid, such as mercury or water for example, or any other suitable material that is readily consumed or transformable to a fluid state by the application of heat. The pattern can be created by computer-generation, especially in the case of wax and stereolithographic patterns, or by manual craftsmanship or by injecting material into a pattern die manufactured by a machining process. Computer generated patterns are preferred for “one-off” or limited volume castings. As will be apparent to those skilled in the art, the pattern may generally be formed by any suitable means useful for producing patterns for conventional investment casting processes.

[0011] The pattern is provided with a refractory coating, preferably by immersing the pattern into an aqueous slurry of ceramic or other refractory material. Of course, methods for providing a refractory coating other than immersion could be used. The slurry may comprise any suitable ceramic or other refractory slurry used in conventional investment casting as described above. Preferably, the pattern is only immersed in the slurry only once and then allowed to dry, thereby providing a refractory coating that has one and only one layer. Consequently, the coating will not be self-supporting. As will be apparent to those skilled in the art, the coating will provide a smooth mold cavity surface once the pattern is consumed and evacuated from the mold. To enhance the surface finish of the resulting casting, the pattern may optionally have a solvent applied to it prior to receiving the refractory coating, especially in the case of a wax pattern. If the optional solvent is applied, a solvent comprising citric acid may be used and can applied by immersing the pattern in the solvent or spraying the pattern with the solvent.

[0012] After the refractory coating is allowed to air dry, the coated pattern is then encased in a casing material that will harden around the pattern without the application of heat to thereby form a casting mold. A suitable chemically-bonded, self-setting sand mixture, well known in the art as “no-bake” sand or a “no-bake” binder, is the preferred casing material. As well known to those skilled in the art, no-bake sand generally comprises a mixture of granular material, such as silica or other sand, and at least two binding substances or catalysts that cause the mixture to harden in the absence of heat when mixed together. The casing material, once hardened, provides a casing or shell around the refractory coated pattern that will support the refractory coating after the pattern is consumed and evacuated from the mold.

[0013] The coated and encased pattern can then be processed generally in accordance with a conventional investment casting processes. In particular, the coated and encased pattern is heated to transform the pattern to a fluid state, and the fluid pattern medium is evacuated from the mold in a conventional manner to provide a refractory-lined mold cavity in the mold. Thereafter, a fluid casting material, such as liquid metal, is supplied into the mold cavity via an inlet or opening in the mold and then allowed to cool to a solid state to thereby form the desired casting.

[0014] An alternative casting method in accordance with this invention comprises following the casting method as described above but without the use of a refractory coating. The pattern is simply encased in a single layer of no-bake material as described above to produce a one-piece, self-supporting shell around the pattern. This alternative method is suitable for applications where the resulting casting does not require the fine surface finish that is provided by use of the refractory coating.

[0015] According to a second aspect of this invention, a consumable pattern is provided with a refractory coating as described above. The coated pattern is then placed in an open container, such as a barrel, canister, or the like, and a granular backing material, such as sand, is packed into the container around the refractory-coated pattern. The backing material is preferably packed around the coated pattern before the refractory material is completely dried so that the backing material adheres to and supports the outer surface of the coating. The pattern may then be heated and evacuated as in a conventional investment casting method and a liquid casting material may then be supplied to the resultant mold cavity via a suitable inlet or opening, such as a tube for example, and then allowed to cool to a solid state.

[0016] Industrial Applicability

[0017] The investment casting methods in accordance with this invention may be used to produce castings much quicker than known investment casting methods that typically require repeatedly coating the pattern with refractory material to produce a rigid, self-supporting refractory shell. This speed advantage is achieved by using a thin, non-self-supporting ceramic or other refractory coating that is supported by a hard casing material or packed backing material. These approached obviate the need for multiple layers of refractory coating and the need for curing the casing or backing material with heat. Moreover, because no heat curing of the mold is required, the cost of a high-temperature curing oven is avoided. The reduction in processing time is particularly suitable for rapidly producing prototypes of parts during product development. Of course, the methods of this invention may also be used for volume production of parts. The methods illustrated herein are effective to produce a suitable mold cavity rapidly and cost-effectively and are particular suited for “one off” or other limited volume castings.

[0018] One contemplated application of the methods described above is in the production of low-volume or personalized jewelry items, such a high school or college rings or pendants or sports championship jewelry (e.g. NCAA, NFL, NBA, NHL, etc.). These items are personalized with names, school names, activities, class year, sports conference, player positions, details of the championship, and other relevant indicia. Because such items are generally personalized, they are not well suited to conventional casting methods that require costly molds. Consumable patterns of the jewelry items may be formed, preferably by automated wax or stereolithographic pattern or model generator, and casting molds may be rapidly and cost-effectively produced using the methods of this invention.

[0019] Another application of the instant invention is in the casting of mold members for use in a conventional casting or molding method.

[0020] Although the presently preferred embodiments of this invention have been described, those skilled in the art will recognize that various modifications may be made without departing from the scope of the appended claims. 

What is claimed is:
 1. A method for producing a casting, comprising the steps of: (a) providing a pattern formed from a medium that is transformable to a fluid state by application of heat; (b) applying a coating of refractory material to the pattern; (c) encasing the refractory-coated pattern in a material that hardens without application of heat; and (d) after the encasing material is hardened, transforming the pattern to a fluid state by applying heat thereto and evacuating the fluid pattern medium from the encasing material, thereby producing a refractory-lined mold cavity in the encasing material; (e) supplying a fluid casting medium into the mold cavity; and (f) permitting the fluid casting medium to achieve a solid state.
 2. The method of claim 1 wherein said encasing material comprises a mixture of granular material and at least two substances that, when mixed together, cause the mixture to harden without application of heat.
 3. The method of claim I wherein said coating comprises one and only one layer of refractory material.
 4. The method of claim 1 further comprising the step, prior to step (b), of applying a solvent to the pattern.
 5. The method of claim 4 wherein said solvent comprises citric acid.
 6. The method of claim 1 wherein the pattern is formed from wax.
 7. The method of claim 6 wherein said wax pattern is formed by an automated wax pattern generator based on a computer model of the object.
 8. The method of claim 1 wherein the pattern is formed by a stereolithographic process.
 9. The method of claim 8 wherein said pattern is formed by an automated steroelithographic pattern generator based on a computer model of the object.
 10. The method of claim 1 wherein said pattern is formed from a tin-bismuth alloy.
 11. The method of claim 1 wherein said pattern is formed from frozen liquid.
 12. The method of claim 1 wherein step (b) comprises immersing said pattern in aqueous slurry of refractory material.
 13. The method of claim 1 wherein said casting comprises a jewelry item.
 14. The method of claim 1 wherein said casting comprises a mold member for use in a casting or molding process.
 15. The method of claim 1 wherein step (d) comprises applying non-curing heat.
 16. The method of claims 1 wherein said coating of refractory material is not self-supporting without said encasing material after the pattern medium is evacuated.
 17. A method for making a casting mold, comprising steps (a), (b), (c), and (d) of claim
 1. 18. A method for producing a casting, comprising the steps of: (a) providing a pattern formed from a medium that is transformable to a fluid state by application of heat; (b) applying a coating of refractory material to the pattern; (c) locating the refractory-coated pattern in a container; (d) packing a free granular backing material in said container around the refractory-coated pattern; (e) transforming the pattern to a fluid state by applying heat thereto and evacuating the fluid pattern medium from the container, thereby producing a refractory-lined mold cavity in the container; (f) supplying a fluid casting medium into the mold cavity; and (g) permitting the fluid casting medium to achieve a solid state.
 19. The method of claim 18 wherein said coating comprises one and only one layer of refractory material.
 20. A method for producing a casting, comprising the steps of: (a) providing a pattern formed from a medium that is transformable to a fluid state by application of heat; (b) encasing the pattern in a single layer of a material that hardens without application of heat to thereby form a one-piece, self-supporting shell around the pattern; (c) after the encasing material is hardened, transforming the pattern to a fluid state by applying heat thereto and evacuating the fluid pattern medium from the encasing material, thereby producing a mold cavity in the encasing material; (d) supplying a fluid casting medium into the mold cavity; and (e) permitting the fluid casting medium to achieve a solid state. 